North Korea has confirmed that it successfully tested a miniaturized nuclear weapon at an underground test site overnight, saying the test was conducted safely and is a response to recent US hostility.

An earthquake was detected in the region with a 4.9 magnitude at the time, which was the first indication that a test may have been conducted. South Korean officials say the earthquake means the explosion was likely of a 6-7 kiloton device, about half of the yield of the first US atomic weapons and far less than the multi-megaton devices in the US and Russian arsenals today.

North Korea promised such a test last month after the UN Security Council approved more sanctions against them for attempting to put a satellite into orbit. The US called the launch a “highly provocative act” and threatened retaliation.

The UN Security Council is to hold an emergency meeting on the test at 9 AM eastern time Tuesday morning, with indications that yet more sanctions against North Korea will be the primary subject. Since isolation of the tiny nation is already near absolute, it is unclear how much more the UN can even theoretically do to them.

The more important reaction will come from neighboring China, whose state media had indicated that they might revoke aid from North Korea if they followed through on the promised test. The loss of Chinese aid would be a huge blow to the nation, as this is one of their last links to the outside world.